The directional solidification of molten material, such as metals, alloys, and eutectics, by unidirectional heat removal from the molten material is well known to impart a directionally oriented grain or microstructure to the solidified castings and is commonly employed in the manufacture of high temperature superalloy turbine blade and vane castings for use in the hot turbine section of gas turbine engines. For example, polycrystalline directionally solidified turbine blades and vanes exhibit a columnar grain structure wherein the grains extend in a preferential loading direction of the blade or vane, while single crystal turbine blades and vanes comprise only a single grain selected and propagated in a preferential loading direction of the blade or vane. Directionally solidified eutectics comprise one or more phase components preferentially aligned in a selected direction of the solidified casting.
In commonly used directional solidification processes, a ceramic investment mold assembly is mounted on a water-cooled copper chill plate U.S. Pat. No. 3,376,915 movable relative to a casting furnace. The mold assembly mounted on the chill plate is positioned in the susceptor of a casting furnace and preheated to a desired elevated temperature for casting by energization of one or more induction coils disposed about the furnace. The induction coils heat a graphite or similar susceptor in the furnace that, in turn, heats the mold assembly to the casting temperature. Molten material, such as a superalloy melt at a selected superheat then is gravity poured into the preheated mold assembly for directional solidification. Directional solidification can be effected by the well known "power down" technique wherein the induction coils are deenergized in controlled manner to establish a desired thermal gradient in the melt in combination with the copper chill plate and/or the "withdrawal" technique wherein the melt-filled mold assembly is withdrawn from the susceptor at a controlled rate to this same end. The Chandley U.S. Pat. No. 3,376,915 discloses the power down and mold withdrawal techniques.
Although directional solidification processing is widely used in the manufacture of columnar grain and single crystal turbine blades and vanes, there is a desire to reduce the overall casting cycle time in order to reduce casting costs. Of special interest in this regard is the manufacture of relatively long single crystal turbine blade and vane castings having relatively massive root sections where a "pigtail" crystal selector passage is provided proximate the bottom of each mold between a lower grain nucleation/growth cavity and a mold cavity thereabove configured to form the desired casting. In particular, the pigtail crystal selector passage typically has a small cross-section and helical configuration in order to select a single crystal in the nucleation zone for propagation through the mold cavity thereabove to form a single crystal casting. However, for relatively long turbine blade and vane castings having relatively massive root sections, the pigtail crystal selector passage is disadvantageous from a heat removal standpoint and can result in relatively long times for achieving directional solidification of the melt in the mold.